1. Field of the Invention
This invention relates to drop-on-demand ink jet printing and more particularly to thermal ink jet printing wherein the ink droplet expulsion mechanics involve fluid inertia of the ink in the vicinity of a collapsing vapor bubble in a pool of ink.
2. Description of the Prior Art
Generally speaking, drop-on-demand ink jet printing systems can be divided into two types. The type using a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle or the type using thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. This latter type is referred to as thermal ink jet printing or bubble ink jet printing and is the subject matter of the present invention. In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al, communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
In U.S. Pat. No. 4,463,359, a thermal ink jet printer is disclosed having one or more ink-filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel at a predetermined distance from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate and those having different colored inks for multicolored printing. In one embodiment, a resistor is located in the center of a relatively short channel having nozles at both end thereof. Another passageway is connected to the open-ended channel and is perpendicular thereto to form a T-shaped structure. Ink is replenished to the open-ended channel from the passgeway by capillary action. Thus, when a bubble is formed in the openended channel, two different recording mediums may be printed simultaneously.
U.S. Pat. No. 4,275,290 to Cielo et al discloses a thermally activated liquid ink printing head having a plurality of orifices in a horizontal wall of an ink reservoir. In operation, an electric current pulse heats selected resistors that surround each orifice and vaporizes the non-conductive ink. The vapor condenses on a recording medium, such as paper, spaced above and parallel to the reservoir wall, causing a dark or colored spot representative of a picture element or pixel. Alternatively, the ink may be forced above the orifice by partial vaporization of the ink, so that the ink is transported by a pressure force provided by vapor bubbles. Instead of partially or completely vaporizing the ink, it can be caused to flow out of the orifices by reduction of the surface tension of the ink. By heating the ink in the orifices, the surface tension coefficient decreases and the meniscus curvature increases, eventually reaching the paper surface and printing a spot. A vibrator can be mounted in the reservoir to apply a fluctuating pressure to the ink. The current pulse to the resistors are coincident with the maximum pressure produced by the vibration.
U.S. Pat. No. 4,251,824 to Hara et al discloses a thermally activated liquid ink jet recording method which involves driving one or a group of heaters to produce vapor bubbles in ink-filled channels of a printhead which expel ink droplets. In FIGS. 7A and 7B, a single resistor is used for each channel to expel drops from nozzles thereof. A plurality of resistors in each channel are shown in FIG. 12 which are sequentially driven to expel droplets. In FIG. 2C, simultaneous driving of varying quantities of resistors in each channel expels droplets of varying diameters.
Japanese Patent Application No. 52-118177 filed Sept. 30, 1977 and published without examination on Apr. 24, 1979 as Laid-Open (Kokai) No. 54-51837 discloses an air bubble produced by a heating element that increases the pressure in the ink chamber which causes ink droplets to be forced out of the chamber through an orifice. The bubble is then cooled by endothermic action and the bubble collapses.
U.S. Pat. No. 4,376,945 to Hara et al discloses a printhead for a thermal ink jet printer wherein various adhesives are used to attach and to hold the printhead parts together. The printhead has one or more ink-filled channels with each have a discharging orifice for ejecting ink droplets at one end, the other end of the channels connect to an ink supply chamber, and a heating element for applying heat energy to the ink in each channel near the orifice. A means for generating mechanical pressure change in the ink flowing into the chamber is provided. The applictaion of the heat energy and the mechanical pressure change is synchronized for the ejection of a droplet. In one embodiment a preliminary biasing heater is used.
U.S. Pat. No. 4,410,899 to Haruta et al discloses a method of forming ink droplets by a heat generator which forms bubbles to expel the droplets, but the bubbles do not fill the channels so that the ink is not totally separated from the nozzle even when the bubbles reach their maximum size.
U.S. Pat. No. 4,409,596 to Ishii discloses a piezoelectric driven ink jet printer in which an intermediate pulses are continuously applied to the ink and a droplet is expelled therefrom whenever a second ejection pulse is combined the intermediate pulse.
IBM Technical Disclosure Bulletin, Vo. 18 No. 4, September 1975 to Fisher et al discloses an ink-on-demand ink jet printer in which jet formation is triggered ultrasonically and the ink reservoir is an ultrasonic cavity which enhances the ultrasonic effects on the meniscus at the orifice. A high-voltage electrode having an orifice therein and an acceleration electrode sandwich the printing medium. A voltage on the order of 2-4 kilovolts is applied to the electrode with the orifice and a voltage of about 7 kilovolts is applied to the acceleration electrode. The voltage from the electrode with the orifice causes a meniscus to be formed at the ink reservoir orifice. When it is desired to expel a droplet, resonant frequency is applied to piezoelectric crystal forming part of the ink reservoir. The combined electrostatic and hydrostatic forces on the ink, when not at resonance, are not sufficient to cause leakage of the ink or formation of a droplet which travels through the electrode orifice and impinges on the printing medium.